Cytoskeletal alterations are known to influence the metastatic success of circulating tumor cells (CTCs). However, studies of the tumor cell cytoskeleton are almost exclusively focused on cells attached to surfaces or extracellular matrix leaving a significant knowledge gap relevant to metastasis. Recent work from the PI's research group has demonstrated that the cytoskeleton of circulating tumor cells responds very differently from cells attached to extracellular matrix. Specifically, free-floating tumor cells produce unique tubulin-based microtentacles (McTNs) that promote the reattachment of CTCs to endothelial cell layers. Imaging in live animals has shown that CTCs reattach to blood vessel walls via a cytoskeletal mechanism that matches McTNs. New data from the PI's lab now demonstrate that McTNs are detectable in tumor cells within hours of their isolation from breast cancer patients during surgery. Moreover, patient tumor cell drug responses can be gauged within minutes. Since McTNs are supported by tubulin, the common breast cancer drug, Paclitaxel (Taxol), actually strengthens McTNs and speeds tumor cell reattachment. Developing cancer drugs aimed at inhibiting attached tumor cell motility by targeting the actin cytoskeleton also enhance McTNs. These results emphasize the importance of determining whether cancer drugs aimed at cell division or the motility of attached tumor cells could inadvertently increase metastasis. Sinc both surgery and neoadjuvant chemotherapy can strongly increase the levels of CTCs, it is important to understand which molecular targets will reduce the metastatic efficiency of CTCs and which may increase metastatic risk. Nearly 90% of human solid tumors arise as carcinomas from epithelial cells, whose large size and rigidity often lead to their fragmentation in narrow capillaries. This restraint on metastatic efficiency could impose a selective pressure for circulating carcinoma cells to develop mechanisms to reattach to blood vessel walls and escape fragmentation. Recent work in the PI's lab has identified that McTNs are increased when the actin cortex beneath the plasma membrane is disrupted via either genetic alterations or chemical treatments. This study will test the hypothesis that genetic or therapeutic dysregulation of the actin cortex induces tubulin McTNs that promote the metastatic reattachment of circulating tumor cells. Predictions of this hypothesis will be tested in the following specific ais: 1) Define the molecular mechanism by which PTEN loss promotes McTNs. 2) Identify the role of Src signaling in McTN extension and tumor metastasis. 3) Examine the role of targeting Rho/ROCK mediated actin contractility on McTN generation. The long-term goals of this project are to define the molecular mechanisms that govern actin disruption to promote McTNs in circulating tumor cells. Extending our understanding of McTN structure and molecular regulation as well as examining McTN incidence and function in tumor cells derived from human breast cancer patients will both identify novel therapeutic targets and reveal potential metastatic risks of current cancer drugs.